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Re: [PATCH v3 02/11] mm: Hardened usercopy

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On Mon, Jul 25, 2016 at 7:03 PM, Michael Ellerman <mpe@ellerman.id.au> wrote:
> Josh Poimboeuf <jpoimboe@redhat.com> writes:
>
>> On Thu, Jul 21, 2016 at 11:34:25AM -0700, Kees Cook wrote:
>>> On Wed, Jul 20, 2016 at 11:52 PM, Michael Ellerman <mpe@ellerman.id.au> wrote:
>>> > Kees Cook <keescook@chromium.org> writes:
>>> >
>>> >> diff --git a/mm/usercopy.c b/mm/usercopy.c
>>> >> new file mode 100644
>>> >> index 000000000000..e4bf4e7ccdf6
>>> >> --- /dev/null
>>> >> +++ b/mm/usercopy.c
>>> >> @@ -0,0 +1,234 @@
>>> > ...
>>> >> +
>>> >> +/*
>>> >> + * Checks if a given pointer and length is contained by the current
>>> >> + * stack frame (if possible).
>>> >> + *
>>> >> + *   0: not at all on the stack
>>> >> + *   1: fully within a valid stack frame
>>> >> + *   2: fully on the stack (when can't do frame-checking)
>>> >> + *   -1: error condition (invalid stack position or bad stack frame)
>>> >> + */
>>> >> +static noinline int check_stack_object(const void *obj, unsigned long len)
>>> >> +{
>>> >> +     const void * const stack = task_stack_page(current);
>>> >> +     const void * const stackend = stack + THREAD_SIZE;
>>> >
>>> > That allows access to the entire stack, including the struct thread_info,
>>> > is that what we want - it seems dangerous? Or did I miss a check
>>> > somewhere else?
>>>
>>> That seems like a nice improvement to make, yeah.
>>>
>>> > We have end_of_stack() which computes the end of the stack taking
>>> > thread_info into account (end being the opposite of your end above).
>>>
>>> Amusingly, the object_is_on_stack() check in sched.h doesn't take
>>> thread_info into account either. :P Regardless, I think using
>>> end_of_stack() may not be best. To tighten the check, I think we could
>>> add this after checking that the object is on the stack:
>>>
>>> #ifdef CONFIG_STACK_GROWSUP
>>>         stackend -= sizeof(struct thread_info);
>>> #else
>>>         stack += sizeof(struct thread_info);
>>> #endif
>>>
>>> e.g. then if the pointer was in the thread_info, the second test would
>>> fail, triggering the protection.
>>
>> FWIW, this won't work right on x86 after Andy's
>> CONFIG_THREAD_INFO_IN_TASK patches get merged.
>
> Yeah. I wonder if it's better for the arch helper to just take the obj and len,
> and work out it's own bounds for the stack using current and whatever makes
> sense on that arch.
>
> It would avoid too much ifdefery in the generic code, and also avoid any
> confusion about whether stackend is the high or low address.
>
> eg. on powerpc we could do:
>
> int noinline arch_within_stack_frames(const void *obj, unsigned long len)
> {
>         void *stack_low  = end_of_stack(current);
>         void *stack_high = task_stack_page(current) + THREAD_SIZE;
>
>
> Whereas arches with STACK_GROWSUP=y could do roughly the reverse, and x86 can do
> whatever it needs to depending on whether the thread_info is on or off stack.
>
> cheers

Yeah, I agree: this should be in the arch code. If the arch can
actually do frame checking, the thread_info (if it exists on the
stack) would already be excluded. But it'd be a nice tightening of the
check.

-Kees

-- 
Kees Cook
Chrome OS & Brillo Security

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[PATCH v3 02/11] mm: Hardened usercopy Kees Cook <keescook@chromium.org> - 2016-07-15 23:50 +0200
  Re: [PATCH v3 02/11] mm: Hardened usercopy Laura Abbott <labbott@redhat.com> - 2016-07-19 03:10 +0200
    Re: [PATCH v3 02/11] mm: Hardened usercopy Kees Cook <keescook@chromium.org> - 2016-07-19 20:50 +0200
      [PATCH] mm: Add is_migrate_cma_page Laura Abbott <labbott@redhat.com> - 2016-07-20 00:10 +0200
        Re: [PATCH] mm: Add is_migrate_cma_page Kees Cook <keescook@chromium.org> - 2016-07-20 00:50 +0200
      Re: [PATCH v3 02/11] mm: Hardened usercopy Balbir Singh <bsingharora@gmail.com> - 2016-07-20 12:30 +0200
        Re: [PATCH v3 02/11] mm: Hardened usercopy Laura Abbott <labbott@redhat.com> - 2016-07-20 17:40 +0200
  Re: [PATCH v3 02/11] mm: Hardened usercopy Laura Abbott <labbott@redhat.com> - 2016-07-19 04:00 +0200
    Re: [PATCH v3 02/11] mm: Hardened usercopy Kees Cook <keescook@chromium.org> - 2016-07-19 21:20 +0200
      Re: [PATCH v3 02/11] mm: Hardened usercopy Kees Cook <keescook@chromium.org> - 2016-07-20 01:00 +0200
  Re: [PATCH v3 02/11] mm: Hardened usercopy Christian Borntraeger <borntraeger@de.ibm.com> - 2016-07-19 11:30 +0200
    Re: [PATCH v3 02/11] mm: Hardened usercopy Kees Cook <keescook@chromium.org> - 2016-07-19 21:40 +0200
      Re: [PATCH v3 02/11] mm: Hardened usercopy Christian Borntraeger <borntraeger@de.ibm.com> - 2016-07-19 22:20 +0200
        Re: [PATCH v3 02/11] mm: Hardened usercopy Kees Cook <keescook@chromium.org> - 2016-07-19 22:40 +0200
          Re: [PATCH v3 02/11] mm: Hardened usercopy Christian Borntraeger <borntraeger@de.ibm.com> - 2016-07-19 22:50 +0200
  Re: [PATCH v3 02/11] mm: Hardened usercopy Kees Cook <keescook@chromium.org> - 2016-07-21 20:40 +0200
    Re: [PATCH v3 02/11] mm: Hardened usercopy Josh Poimboeuf <jpoimboe@redhat.com> - 2016-07-22 19:50 +0200
      RE: [PATCH v3 02/11] mm: Hardened usercopy David Laight <David.Laight@ACULAB.COM> - 2016-07-25 11:30 +0200
        RE: [PATCH v3 02/11] mm: Hardened usercopy Michael Ellerman <mpe@ellerman.id.au> - 2016-07-26 04:10 +0200
      Re: [PATCH v3 02/11] mm: Hardened usercopy Kees Cook <keescook@chromium.org> - 2016-07-26 06:50 +0200

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